In microlithography it is generally desirable to keep the position and the geometry of the components (e.g., the optical elements such as lenses, mirrors and gratings) of an imaging device unchanged during operation to the highest possible extent in order to achieve a correspondingly high imaging quality. The tough requirements with respect to accuracy lying in a microscopic range in the area of a few nanometers are none the less a consequence of the permanent need to reduce the resolution of the optical systems used in fabricating microelectronic circuitry in order to push forward miniaturization of the microelectronic circuitry to be produced.
In order to achieve an increased resolution either the wavelength of light used may be reduced as it is the case with systems working in the extreme ultraviolet (EUV) range at working wavelengths in the area of 13 nm or the numerical aperture of the projection system used may be increased. One possibility to remarkably increase the numerical aperture above the value 1 is realized in so-called immersion systems, wherein an immersion medium having a refractive index larger than 1 is placed between an immersion element of the projection system and the substrate to be exposed. A further increase in the numerical aperture is possible with optical elements having a particularly high refractive index.
In a so-called single immersion system, the immersion element (i.e. the optical element at least in part contacting the immersion medium in the immersed state) typically is the last optical element located closest to the substrate to be exposed. Here, the immersion medium typically contacts this last optical element and the substrate. In a so-called double immersion system, the immersion element does not necessarily have to be the last optical element, i.e., the optical element located closest to the substrate. In such double or multiple immersion systems, and immersion element may also be separated from the substrate by one or more further optical elements. In this case, the immersion medium the immersion element is at least partly immersed in may be placed, for example, between two optical elements of the optical system.
With the reduction of the working wavelength as well as with the increase of the numerical aperture not only the requirements with respect to the positioning accuracy and the dimensional accuracy of the optical elements used become more strict throughout the entire operation. Of course, the requirements with respect to the minimization of imaging errors of the entire optical arrangement increase as well.
The temperature distribution within the optical elements used and the deformation of the respective optical element eventually resulting from the temperature distribution as well as an eventual temperature related variation of the refractive index of the respective optical element can be important in this context.
Various approaches taken to actively counteract heating of a mirror (e.g., in an EUV system) resulting from the incident light and to keep a temperature captured at a given location within the mirror actively within given limits. For example, one can use a temperature adjustment device located centrally on the backside of a mirror including Peltier elements or the like to provide targeted cooling. This solution, on the one hand, can have the disadvantage that it is not suitable for use with refractive optical elements as they are used in particular with the immersion systems mentioned above since the central temperature adjustment device would then cover the area optically used. On the other hand, only the temperature of a single location within the mirror is generally reliably controlled in a more or less stationary state considering the light energy absorbed by the mirror. Further thermal influences of the environment, in particular non-stationary and/or locally varying thermal influences as they may be introduced by an immersion medium and as they may cause dynamic and local fluctuations in the temperature distribution within the mirror, respectively, remain unconsidered. See, e.g., EP 1 477 853 A2 (to Sakamoto), the entire disclosure of which is incorporated herein by reference.